A conventional device of this kind is disclosed, for example, in Japanese Patent Laid-Open Publication No. 11-132014. This device includes a rotor, a housing which can rotate relative to the rotor, a projecting portion which is formed on the housing so as to slide on the outer circumference of the rotor, a fluid chamber which is defined between the rotor and the housing, a vane which is provided on the rotor and which divides the fluid chamber into a retard angle chamber and an advance angle chamber and a torsion coil spring for urging the rotor relative to the housing in the advance angle direction in which the volume of the retard angle chamber decreases and the volume of the advance angle chamber increases. The torsion coil spring is provided considering the force which operates the rotor relative to the housing in the retard angle direction due to the fluctuation torque constantly operating to the cam shaft during the running of the engine. The torsion coil spring improves the response of the rotation of the rotor toward the advance side. One end of the torsion coil spring is engaged with a first groove which is formed on a plate connected to the housing and the other end thereof is engaged with a second groove formed on the rotor.
In the above prior art, a first hook portion which is extended in the axial direction of a coil portion of the torsion spring is formed on one end of the torsion spring. The first hook portion is inserted into a first hook engaging hole formed on a groove bottom of the first groove and is engaged with the first hook engaging hole. A second hook portion which is extended in the axial direction of the coil portion of the torsion spring is formed on the other end of the torsion spring. The second hook portion is inserted into a second hook engaging hole formed on a groove bottom of the second groove and is engaged with the second hook engaging hole.
In the above prior device, in order to hold the position of the torsion coil spring, the coil portion located at one end of the torsion coil spring is engaged with a projection and a spiral groove which are formed on the plate. Therefore, the inner and outer diameter of the coil spring changes by the change of the twisting angle of the torsion coil spring during the operation of the valve timing control device. As a result, the coil portion contacts frictionally with the projection and the spiral groove and therefore the torsion coil spring can not apply the desired twisting torque.